Between 7 and 20 August 1999 a Public Health Department was notified of seven cases of Escherichia coli O157 infection. Phage typing demonstrated that four were related and three unrelated. The investigation centred around the three cases of E. coli O157 phage type 21/28 who visited the same beach during the last week of July. The fourth related infection was identified as a result of the investigation. Although the investigation failed to identify a source of infection, interviews with the families of the four related cases suggested that they had occupied the same part of the beach on the same day and so it is possible that a contaminated area of beach was the source. We believe this to be the first outbreak of E. coli O157 to be associated with a bathing beach.